1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) for driving pixels by a thin film transistor (TFT) to perform liquid crystal display operation, and, more particularly, to a reflection type liquid crystal display utilizing poly silicon which is prepared using in a low-temperature process and to a projector employing this reflection type liquid crystal display.
2. Description of the Related Art
A liquid crystal display having a liquid crystal sealed between a pair of substrates for applying a voltage to the liquid crystal to perform desirable display operation has an advantage that it is small-sized, thin, and has reduced power consumption. For these reasons, liquid crystal displays are now used in a variety of OA and AV devices, portable or on-board information devices, and the like. It has also been proposed a transmission type liquid crystal display be used as a projector.
An active-matrix liquid crystal display using a thin film transistor (hereinafter referred to as a TFT) as a switching element for driving each liquid crystal pixel can selectively drive each TFT to select a particular liquid crystal pixel. Therefore, image display with no cross talk and with a higher resolution can be obtained.
Two known TFTs used in liquid crystal displays are amorphous silicon TFTs using amorphous silicon for an active layer and a poly silicon TFT using more highly mobile poly silicon for the active layer. The amorphous silicon TFT is most common in large-sized displays and the like because it can be formed over a large area through a low-temperature process. On the other hand, the poly silicon TFT has a higher mobility than the amorphous silicon and can form an element in a self-aligned manner. Therefore, the poly silicon TFT can more easily reduce TFT and pixel area than can the amorphous silicon TFT, so that a display having a higher resolution can more easily be manufactured. With the use of the poly silicon, the TFT can easily have a CMOS structure. For this reason, a driver TFT for driving a display section TFT can be formed on the same substrate at almost the same processes as the display section TFT.
As stated above, the poly silicon TFT has attractive characteristics and can include a driver on its substrate. The poly silicon is formed by polycrystallizing amorphous silicon in a high-temperature process. As the amorphous silicon is exposed to a high temperature during the process, an inexpensive glass substrate cannot be used for the substrate. For this reason, the poly silicon TFT faces a practical problem with respect to cost.
However, polycrystallization technology using an annealing treatment such as laser annealing, lamp annealing, or the like has been improved. Consequently, it has become possible to manufacture poly silicon in the low-temperature (below 600xc2x0 C.) process.
By forming a poly silicon TFT in a low-temperature process, an inexpensive glass substrate can be developed for use as a TFT substrate. Cost can be reduced and, furthermore, effective area can be increased. As a result, a poly silicon TFT (hereinafter referred to as a low-temperature poly silicon TFT) formed in a low-temperature process has practical uses.
Although low-temperature poly silicon TFTs have been put into practical use, a liquid crystal material and a panel structure which are optimum for exhibiting the characteristics of the low-temperature poly silicon TFT for a liquid crystal display at the maximum and for enhancing the characteristics have yet to be developed. Commonly, a material and a structure which had been used in a conventional amorphous silicon TFT liquid crystal display are employed without adaptation. Consequently, there is a problem that the characteristics of the poly silicon TFT cannot be exhibited fully.
As mentioned above, a liquid crystal display can be utilized in a projector. More specifically, light passing or reflected through the liquid crystal display is controlled by the liquid crystal display, and is projected onto a screen. Consequently, an image is displayed on that screen. With the use of a reflection type liquid crystal display, the size of the projector can be comparatively reduced. In an ordinary liquid crystal display, however, a viewing angle is small. Therefore, there has been a problem that it is difficult to obtain sufficient reflected light.
It is an object of the present invention to obtain a reflection type liquid crystal enabling the best use of the characteristics of a low-temperature poly silicon TFT, and a projector utilizing the reflection type liquid crystal display.
In order to attain the above-mentioned object, the present invention has the following features.
The present invention provides an active-matrix type liquid crystal display comprising a plurality of pixel electrodes in a matrix on a first substrate, a thin film transistor formed on the first substrate to be connected with the corresponding pixel electrodes, and a liquid crystal layer interposed between the pixel electrodes provided on the first substrate and a common electrode on a second substrate opposed to the first substrate, the liquid crystal layer being driven by each pixel electrode to perform display operation, wherein the thin film transistor uses a poly silicon layer for an active layer, an initial direction of each liquid crystal molecule of the liquid crystal layer interposed between the first and second substrates is controlled in a nearly vertical direction with respect to the pixel electrode, a liquid crystal molecule with a molecular structure having fluorine side chain is selected as a liquid crystal material to be used for the liquid crystal layer, and the pixel electrode is formed of a reflection material and is used as a reflection layer. Preferably, the poly silicon layer is formed at a low temperature.
The present invention allows the realization of a projector comprising a first polarizing plate for subjecting light emitted from a light source to predetermined polarization and transmitting the polarized light, a liquid crystal display for receiving and reflecting the light transmitted through the first polarizing plate in an oblique direction, and a second polarizing plate for transmitting the reflected light emitted from the liquid crystal display in a direction orthogonal to the first polarizing plate, the light transmitted through the second polarizing plate being projected onto a screen, wherein the liquid crystal display to be used has the above-mentioned structure.
Furthermore, the present invention provides a projector comprising a polarization beam splitter for receiving light incident from a light source in a direction that forms an angle of approximately 45xc2x0 with the surface, and for transmitting light polarized in a predetermined direction, and reflecting light polarized in a direction orthogonal to the predetermined direction, a mirror for reflecting the polarized light transmitted through the polarization beam splitter and for causing the reflected light to be incident on the polarization beam splitter again, and a liquid crystal display for reflecting the light reflected by the polarization beam splitter and for causing the reflected light to be incident on the polarization beam splitter again, the light reflected by the liquid crystal display and transmitted through the polarization beam splitter being projected onto a screen, wherein the liquid crystal display to be used has the above-mentioned structure.
As a liquid crystal material to be used for the liquid crystal layer, preferably, at least one kind of liquid crystal molecule in Formulas (1) to (6) including a molecular structure having fluorine on a side chain is selected from materials having the molecular structures indicated by the Formulas (1) to (6). 
It may be preferable that an electrode-free portion for controlling the alignment of the liquid crystal be provided as an alignment control window in a predetermined region of the common electrode provided on the second substrate which is opposed to the pixel electrode, and that a plurality of vertical alignment regions having different tilt azimuths be formed in each pixel electrode region while changing the alignment of the liquid crystal molecule into a tilted vertical alignment.
It may also be preferable that the first substrate have a flattened interlayer insulator film formed thereon to cover the thin film transistor which is provided on the first substrate, and that the pixel electrodes formed on the flattened interlayer insulator film cover at least a region where the thin film transistor is to be formed.
It also may be preferable that a liquid crystal material to be used for the liquid crystal layer have a negative dielectric anisotropy, and that the vertical alignment of the liquid crystal layer be controlled without a rubbing step using a vertical alignment film formed to cover the common electrode and the pixel electrodes, the alignment control window provided on the common electrode, and a voltage applied to respective pixel electrodes.
According to the present invention, thus, the liquid crystal display can have a great viewing angle and, when the liquid crystal display is used as a reflection type liquid crystal display for a projector, sufficient light can be reflected.